Control device for internal combustion engine

ABSTRACT

An operating range boundary for switching a cam for driving an intake valve (drive cam) is changed in a direction of increasing an engine load if a target EGR rate is predicted to increase across the contour line of the EGR rate during an acceleration operation. By changing to such a high load direction, a range in which a large cam is selected is enlarged. That is, switching of the drive cam from the large cam to a small cam is delayed.

CROSS-REFERENCE TO RELATED APPLICATION

The present disclosure claims priority under 35 U.S.C. § 119 to Japanese Patent Applications No. 2017-26422, filed on Feb. 15, 2017. The contents of these applications are incorporated herein by reference in their entirety.

TECHNICAL FIELD

The present disclosure relates to a control device for an internal combustion engine.

BACKGROUND

JP 2013-72342 A discloses a control device for an internal combustion engine which is configured to control an engine in which a part of exhaust gas as external EGR gas is recirculated from an exhaust system to an intake system. In such a conventional control device, an opening degree of an EGR valve is controlled based on a map defining a relationship between an operating range defined by engine speed and engine load and a target amount of external EGR gas (hereinafter referred to as a “target EGR amount”). In the map, the operating ranges are partitioned by contour lines of the target EGR amount. According to the map, the target EGR amount is set to a highest value in a partitioned range including a middle-engine-speed-and-middle-engine-load range, and decreases from this partitioned range toward a peripheral partitioned range.

The target EGR amounts in the map are obtained by an experiment or simulation performed in advance. According to the map, an actual external EGR amount (hereinafter also referred to as an “actual EGR amount”) can be maintained at an optimum value during a steady operation in which the engine operating state stays in a partitioned range having an equal target EGR amount. On the other hand, the actual EGR amount is largely affected by time lag during a transition operation in which the engine operating state is transferred across the contour line of the target EGR amount. When the engine operating state is transferred from a partitioned range with large target EGR amount to a partitioned range with low target EGR amount, for example, the large influence by time lag causes a period during which the actual EGR amount becomes excessive with respect to the target EGR amount. Then, the combustion in this excessive period tends to deteriorate easily. Accordingly, a countermeasure against such deterioration of combustion during the transition operation is needed.

The present disclosure addresses the above described problem, and an object of the present disclosure is to suppress to occur deterioration in combustion when the engine operating range and the engine operating state is transferred from the partitioned range with high target amount to the partitioned range with low target amount, in a case where an opening degree of an EGR valve is controlled based on the map defining a relationship between the target amount of external EGR gas and the engine operating range.

A first aspect of the present disclosure is a control device for an internal combustion engine which is configured to control an engine in which a part of exhaust gas as external EGR gas is recirculated from an exhaust system to an intake system,

wherein the control device comprising:

an EGR map the defines a relationship between an operating range defined by engine speed and engine load and a target value of external EGR rate, and has a predetermined partitioned range in which the target value is set to a highest value; and

an operating angle map that defines a relationship between the operating range and an operating angle of an intake cam for driving an intake valve of the engine,

wherein the operating angle map being set so that

a large operating angle is selected in a first region including a region corresponding to the predetermined partitioned range, the large operating angle being capable of closing the intake valve in a first crank angle section, and

a small operating angle is selected in a second region in which the engine load is higher than that of the first region, the small operating angle being capable of closing the intake valve in a second crank angle section that is located nearer to a bottom dead center side than the first crank angle section,

wherein the control device is configured to:

select the operating angle in accordance with the operating angle map when it is predicted that the engine operating state stays in a partitioned range having the equal target value in the EGR map; and

when it is predicted that the engine operating state is transferred from a partitioned range with the high target value to a partitioned range with the low target value in the EGR map, in a case where the engine operating state is transferred in a direction of increasing the engine load, change a boundary between the first region and the second region in a direction of increasing the engine load, and then select the operating angle in accordance with the operating angle map.

A second aspect of the present disclosure is the control device for an internal combustion engine according to the first aspect,

wherein the control device is further configured to, when changing the boundary in the direction of increasing the engine load, increase a degree of a change of the boundary as a change rate of an accelerator opening degree of the engine becomes larger.

A third aspect of the present disclosure is the control combustion engine according to the first aspect,

wherein the control device is further configured to when changing the boundary in the direction of increasing the engine load:

calculate a time interval from a change point of the target value set in accordance with the EGR map to an increase starting point of an actual external EGR rate; and

increase the degree of the change of the boundary as the time interval is larger.

A fourth aspect of the present disclosure is the control device for an internal combustion engine according to any one of the first to third aspects,

wherein the engine comprising a turbocharger including a compressor and a turbine, and

the external EGR gas is recirculated from a downstream side of the turbine to an upstream side of the compressor.

According to the first aspect, when it is predicted that the engine operating state is transferred from a partitioned range with high target value of the external EGR rate to a partitioned range with low target value of the external EGR rate in the EGR map, in a case where the engine operating state is transferred in a direction of increasing the engine load, the boundary between the first region and the second region is changed in a direction of increasing the engine load, and then the operating angle can be selected in accordance with the operating angle map. When the boundary is changed in the direction of increasing the engine load, the first region in which the large operating angle is selected is enlarged. Accordingly, it is possible to continue to select the large operating angle even in the period in which the small operating angle is originally selected. Here, when selecting the large operating angle, the turbulence in the cylinder is larger than in a case of selecting the small operating angle. Therefore, according to the first aspect, it is possible to suppress deterioration of combustion in the period during which the actual EGR rate becomes excessive with respect to the target EGR rate.

According to the second aspect, the degree of the change of the boundary in the direction of increasing the engine load can be changed as the change rate of the accelerator opening degree becomes larger. Accordingly, it is possible to suppress deterioration of combustion in the period during which the actual EGR rate becomes excessive in accordance with the change rate of the accelerator opening degree.

According to the third aspect, the time lag from the change point of the target value of the external EGR rate set in accordance with the EGR map to the increase starting point of the actual value of the external EGR rate is directly calculated, and the degree of the change of the boundary can be increased as the time lag is larger. Therefore, it is possible to suppress deterioration of combustion in the period during which the actual EGR rate becomes excessive.

According to the fourth aspect, it is possible to suppress deterioration of combustion of the engine provided with an LPL-EGR device in the period during which the actual EGR rate becomes excessive.

BRIEF DESCRIPTION OF DRAWINGS

FIG. 1 is a diagram illustrating a configuration example of a system according to a first embodiment of the present disclosure;

FIG. 2 is an exemplary graph describing cam profiles of two types of intake cams that are provided in the system according to the first embodiment of the present disclosure;

FIG. 3 is an exemplary graph showing a relationship between an engine operating range and a target EGR rate;

FIG. 4 is an exemplary graph showing a relationship between the engine operating range and the cam for driving the intake valve;

FIG. 5 is an exemplary graph describing an intake valve closing timing;

FIG. 6 is an exemplary graph showing a change in the engine operating state during a transition operation (acceleration operation);

FIG. 7 is a graph describing a method of changing a switching boundary in the first embodiment of the present disclosure;

FIG. 8 is time charts describing a relationship between transitions of an accelerator opening degree, an engine load, and an external EGR rate, and a drive cam, respectively, when an operating point is transferred from PA to PB as shown in FIG. 6;

FIG. 9 is time charts describing transitions of the accelerator opening degrees, the engine load and the external EGR rates during the acceleration operation, respectively;

FIG. 10 is a graph describing a method of adjusting a switching boundary in a second embodiment of the present disclosure; and

FIG. 11 is a graph describing a method of adjusting the switching boundary in a third embodiment of the present disclosure.

DESCRIPTION OF EMBODIMENTS

Hereinafter, embodiments of the present disclosure are described based on the drawings. It is to be noted that common elements in each figure are designated by the same reference numerals, and duplicated description thereof are omitted herein. It is also to be noted that the following embodiments do not limit the present disclosure.

First Embodiment

A first embodiment of the present disclosure is described with reference to FIGS. 1 to 8.

[Description of System Configuration Example]

FIG. 1 is a diagram illustrating a configuration example of a system according to the first embodiment of the present disclosure. The system illustrated in FIG. 1 is a system for an internal combustion engine mounted in a vehicle. The system illustrated in FIG. 1 includes an internal combustion engine 10 as a driving source. The internal combustion engine 10 is a four-stroke reciprocating engine, and also an in-line three cylinder engine. It is to be noted that the number and arrangement of cylinders of the internal combustion engine 10 are not particularly limited to the above-described number and arrangement. Each cylinder of the interval combustion engine 10 communicates with an intake pipe 12 and an exhaust pipe 14.

An intake system of the internal combustion engine 10 is described. An air cleaner 16 is attached in the vicinity of an inlet of the intake pipe 12. A compressor 18 a of a turbocharger 18 is provided downstream of the air cleaner 16. The compressor 18 a is driven by rotation of a turbine 18 b that is provided in the exhaust pipe 14, to compress intake air. An electronic control throttle valve 20 is provided downstream of the compressor 18 a. An intake manifold 22 that is connected to intake ports of each cylinder is provided downstream of the throttle valve 20. A water-cooled type intercooler 24 is incorporated in the intake manifold 22. Intake air flowing in the intercooler 24 is cooled by heat exchange with cooling water flowing in a cooling pipe 26.

Next, an exhaust system of the internal combustion engine 10 is described. The turbine 18 b of the turbocharger 18 is attached to the exhaust pipe 14. The turbine 18 b is connected to the compressor 18 a. The turbine 18 b is rotated by energy of exhaust gas flowing in the exhaust pipe 14. A bypass pipe 28 that bypasses the turbine 18 b is provided in a middle of the exhaust pipe 14. A WGV (waste gate valve) 30 is provided in the bypass pipe 28. The WGV 30 is opened when an exhaust pipe pressure (back pressure) on an upstream side of the turbine 18 b is higher than a predetermined value. When the WGV 30 is opened, a part of exhaust gas flowing in the upstream side of the turbine 18 b flows into the downstream side of the turbine 18 b through the bypass pipe 28. Catalysts 32 and 34 for cleaning exhaust gas are provided in the downstream side of the turbine 18 b.

Next, an EGR system for the internal combustion engine 10 is described. The internal combustion engine 10 includes an LPL-EGR (low pressure loop-EGR) device 36. The LPL-EGR device 36 includes an EGR pipe 38 that connects the exhaust pipe 14 between the catalysts 32 and 34, and the intake pipe 12 on the upstream side of the compressor 18 a. A water-cooled type EGR cooler 40 is provided in the middle of the EGR pipe 38. Exhaust gas flowing in the EGR cooler 40 (i.e., external EGR gas) is cooled by heat exchange with cooling water flowing in a cooling pipe 42. An electronic control EGR valve 44 is provided on the downstream side of the EGR cooler 40. A change of an opening degree of the EGR valve 44 causes a change of a flow amount of the external EGR gas that flows from the EGR pipe 38 into the intake pipe 12. When the opening degree of the EGR valve 44 becomes larger, an external EGR rate increases.

Next, a valve system for the internal combustion engine 10 is described. FIG. 2 is an exemplary graph describing cam profiles (meaning at least one of a lift amount and an operating angle, the same shall apply hereinafter) of two types of intake cams that are provided in the system according to the first embodiment of the present disclosure. As illustrated in FIG. 2, the system according to the first embodiment includes a large cam and a small cam as the two types of intake cams. The small cam has an operating angle and a lift amount that are smaller than those of the large cam. The large cam and the small cam are carried on a camshaft that rotates in synchronization with a crankshaft. Two pair of large and small cams are carried on one cylinder because two intake valves are provided per cylinder. However, the number of intake valves per cylinder in the present disclosure may be one, or three or more. One of the intake earns is used as an intake cam for driving the intake valve (hereinafter also referred to as “drive cam”). The drive cam is switched between the large cam and the small cam by a switching operation of a switching mechanism.

The camshaft carrying the large cam and the small cam is provided with a VVT (variable valve timing mechanism). The VVT is a mechanism that varies a rotational phase difference of the camshaft with respect to the crankshaft thereby to vary a valve opening characteristic of the intake valve. The VVT includes a housing that is connected to the crankshaft through a timing chain or the like, and a valve body that is provided in the housing and attached to an end portion of the camshaft. Hydraulic pressure is supplied into a hydraulic chamber partitioned by the housing and the valve body, to thereby enable the valve body to be relatively rotated with respect to the housing, and further enable the rotational phase difference of the camshaft with respect to the crankshaft to be varied. The hydraulic pressure supplied to the VVT is controlled by a hydraulic pressure control valve provided in a hydraulic pressure supply line. A system of the VVT is known, and a configuration of the system is not limited in the present disclosure, and thus the further descriptions of the VVT are omitted.

Returning to FIG. 1, the configuration example of the system is continuously described. The system illustrated in FIG. 1 includes an ECU (Electronic Control Unit) 50 as a control device. The ECU 50 includes a RAM (Random Access Memory), a ROM (Read Only Memory), a CPU (microprocessor) and the like. The ECU 50 takes in and processes signals from various sensors mounted in a vehicle. The various sensors include an air flow meter 52, a crank angle sensor 54, an accelerator opening degree sensor 56, and a supercharging pressure sensor 58. The air flow meter 52 is provided in the vicinity of the air cleaner 16, and detects an intake air amount. The crank angle sensor 54 outputs a signal according to a rotation angle of the crankshaft. The accelerator opening degree sensor 56 detects a step-on amount of an accelerator pedal by a driver. The supercharging pressure sensor 58 detects an intake pipe pressure (supercharging pressure) on the upstream side of the throttle valve 20. The ECU 50 takes in and processes the signals from the various sensors to operate various actuators in accordance with a predetermined control program. The various actuators include the above-described throttle valve 20 and WGV 30. The various actuators also include a VVT 60 and a cam switching mechanism 62.

[Characteristic Control in First Embodiment]

FIG. 3 is an exemplary graph showing a relationship between an engine operating range and a target EGR rate. The relationship in FIG. 3 is created based on a simulation performed in advance. It is to be noted that the target EGR rate means a target value obtained by dividing an external EGR amount by an intake air amount, or can be also referred to as a value obtained by dividing the above-described target EGR amount by the intake air amount. Among ranges partitioned by contour lines shown in FIG. 3, the target FOR rate is set to the highest value in the partitioned range including a middle-engine-speed-and-middle-engine-load range. Thus, the external EGR rate is increased in the middle-engine-speed-and-middle-engine-load range that is used with particularly high frequency, to decrease an intake air temperature, thereby improving the heat efficiency. The target EGR rate is set to a lower value in an operating range for which the frequency of use becomes relatively lower. Specifically, the target EGR rate is set to a lower value in the partitioned ranges including a high engine load range and a low engine load range compared with a value in the partitioned ranges including a middle engine load range. Similarly, the target EGR rate is set to a lower value in the partitioned ranges including a high engine speed range and a low engine speed range compared with a value in the partitioned ranges including a middle engine speed range. In the first embodiment, the relationship shown in FIG. 3 is stored in the ROM of the ECU as a map, and an actual operating state is applied to the map to thereby control an opening degree of the EGR valve.

In the first embodiment, the engine is controlled by combining an intake valve closing timing with the above-described target EGR rate. FIG. 4 is an exemplary graph showing a relationship between the engine operating range and the cam for driving the intake valve. As shown in FIG. 4, the large cam is selected in the middle-engine-speed-and-middle-engine-load range and the low-engine-speed-and-low-engine-load range, and the small cam is selected in the high-engine-speed-and-high-engine-load range. In the first embodiment, the relationship shown in FIG. 4 is stored in the ROM of the ECU as a map, and an actual operating state is applied to the map to thereby control the switching operation of the cam switching mechanism.

FIG. 5 is an exemplary graph describing an intake valve closing timing. As shown in FIG. 5, when a drive cam is a large cam, the intake valve is closed in a crank angle section CA₁ retarded with respect to a bottom dead center (ABDC=0). On the other hand, when a drive cam is a small cam, the intake valve is early closed in a crank angle section CA₂ including the bottom dead center. Widths of the crank angle sections CA₁, CA₂ as shown in FIG. 5 are provided to change the intake valve closing timing by the VVT. However, when the large cam is selected as the drive earn to increase the engine output, the crank angle section CA₁ is set so as to include a crank angle at which the suction efficiency is maximized. On the other hand, when the small cam having a small lift amount is selected as the drive earn, the crank angle section CA₂ is set so as not to include the crank angle at which the suction efficiency is maximized. It is to be noted that the suction efficiency shown in FIG. 5 can be obtained under operating conditions in which the engine speed is fixed, for example.

When the EGR valve is controlled based on the relationship shown in FIG. 3, an actual external EGR rate (hereinafter also referred to as an “actual EGR rate”) can be controlled to an optimum value during a steady operation in which the engine operating state stays in a partitioned range having an equal target EGR rate. On the other hand, the actual EGR rate is largely affected by time lag during a transition operation in which the engine operating state is transferred across the contour line of the target FOR rate. This problem is described with reference to FIG. 6. FIG. 6 is an exemplary graph showing a change in the engine operating state during the transition operation. FIG. 6 is a graph in which the relationships shown in FIG. 3 and FIG. 4 are combined. FIG. 6 shows an example of a change in the engine operating state during an acceleration operation. This example assumes that the engine operating state is changed from an operating point PA to an operating point PB. When the operating point is transferred from PA to PB, the operating point is transferred from a partition range R₁ through a partition range R₂ to a partitioned range R₃.

The target EGR rate is set to the highest value in the partitioned range R₁, and becomes lower in order of the partitioned ranges R₂, R₃, and R₄. Thus, when the operating point is transferred from PA to PB, the target EGR rate continues to decrease. However, the time lag produces a period during which the actual EGR rate becomes excessive with respect to the target EGR rate. When such an excessive period of the actual EGR rate occurs, combustion state in the cylinder tends to became unstable. Furthermore, when the drive cam is switched from the large cam to the small cam during the excessive period, turbulence in the cylinder is reduced. Therefore, deterioration of the combustion state in the cylinder cannot be avoided.

In consideration of these reasons, in the first embodiment, an operating range boundary for switching the drive cam (hereinafter also referred to as a “switching boundary”) is changed in a direction of increasing the engine load if the target EGR rate is predicted to increase across the contour line shown in FIG. 3 during the acceleration operation. FIG. 7 is a graph describing a method of changing the switching boundary in the first embodiment of the present disclosure. Operating points PA, PB and partitioned ranges R₁ to R₄ that are shown in FIG. 7 correspond to the operating points PA, PB and partitioned ranges R₁ to R₄ that are shown in FIG. 6, respectively. As can be seen from comparing FIG. 6 and FIG. 7, a switching boundary in FIG. 7 is changed in a higher load direction than that in FIG. 6. By changing to such a high load direction, a range in Which the large cam is selected is enlarged. That is, switching of the drive cam from the large earn to the small cam is delayed. Therefore, it is possible to suppress deterioration of the combustion state in the cylinder due to the switch of the drive cam during the excessive period.

FIG. 8 is time charts describing a relationship between transitions of an accelerator opening degree, an engine load, and an external EGR rate, and a drive, respectively, when the operating point is transferred from PA to PB as shown in FIG. 6. As shown in FIG. 8, the accelerator opening degree starts to increase at a time t₁. When the accelerator opening degree increases, the engine load increases from LA to LB. It is to be noted that the engine load. LA shown in FIG. 8 represents the engine load at the operating point PA shown in FIG. 6, and the engine load LB represents the engine load at the operating point PB shown in FIG. 6.

Since the target EGR rate follows the relationship shown in FIG. 3, when the engine load increases after the time t₁, the target EGR rate decreases. When the target EGR rate decreases, the EGR valve opening degree becomes smaller, and the actual EGR rate also decreases. The actual EGR rate starts to increase after the time t₁ because this is affected by the above-described time lag. It is to be noted that the target EGR rate starts to decrease after reaching the maximum valve because the operating point is transferred from the partitioned range R₁ to the partitioned range R₂ as shown in FIG.

The ECU predicts whether the target EGR rate increases across the contour line shown in FIG. 6 based on a change rate of the accelerator opening degree after the time t₁. For example, the ECU stores in the ROM a threshold set in advance based on intervals between the contour lines of the target EGR to compare between the threshold and the change rate of the accelerator opening rate. When the ECU determines that the change rate of the accelerator opening degree exceeds the threshold after the time t₁, the ECU predicts that the target EGR rate increases across the contour line shown in FIG. 3.

In FIG. 8, it is predicted at the time t₂ that the target EGR rate increases across the contour line shown in FIG. 3. When the drive cam is switched, at the time t₂, based on the relationship shown in FIG. 4 from the large cam to the small cam, the combustion state in the cylinder easily deteriorates. In this respect, in the first embodiment, since the switching boundary is changed in the direction of increasing the engine load, it is possible to continue to select the large cam until a time t₃ being later than the time t₂. Therefore, it is possible to suppress the deterioration of the combustion state in the cylinder during the excessive period.

In the above-described first embodiment, the map defining the relationship shown in FIG. 3 corresponds to an “EGR map” in a first aspect. The map defining the relationship shown in FIG. 4 corresponds to an “operating angle map” of the first aspect. The partitioned range R₁ shown in FIG. 6 corresponds to a “predetermined partitioned range” of the first aspect. The crank angle section CA₁ described in FIG. 5 corresponds to a “first crank angle section” of the first aspect. The crank angle section CA₂ corresponds to a “second crank angle section” of the first aspect.

Second Embodiment

A second embodiment of the present disclosure is described with reference to FIGS. 9 to 10.

[Characteristic Control in Second Embodiment]

In the above-described first embodiment, when it is predicted during the acceleration operation that the target EGR rate increases across the contour line shown in FIG. 3, the switching boundary is changed in the direction of increasing the engine load. The second embodiment takes a change rate of an accelerator opening degree during the acceleration operation into consideration.

FIG. 9 is time charts describing transitions of the accelerator opening degrees, the engine load and the external EGR rates during the acceleration operation, respectively. FIG. 9 shows a case of rapid acceleration in which the rate of change in the accelerator opening degree is large and a case of slow acceleration in which the rate of change in the accelerator opening degree is small. As shown in FIG. 9, in the case of rapid acceleration, the increase in the engine load is slower than that in the case of slow acceleration, and the decrease in the target EGR rate is also slower than that in the case of slow acceleration.

In the second embodiment, therefore, a position of the switching boundary is adjusted in accordance with the change rate of the accelerator opening degree during the acceleration operation. FIG. 10 is a graph describing a method of adjusting the switching boundary in the second embodiment of the present disclosure. As shown in FIG. 10, in the second embodiment, the engine load at the switching boundary is set to a higher value as the change rate becomes larger. That is, the degree of the change of the switching boundary is increased as the change rate becomes larger. Therefore, it is possible to suppress the deterioration of the combustion state in the cylinder during the excessive period in accordance with the change rate of the accelerator opening degree. It is to be noted that the engine load value when the change rate of the accelerator opening degree equals zero as shown in FIG. 10 corresponds to the engine load value at the switching boundary during the steady operation shown in FIG. 4.

Third Embodiment

A third embodiment of the present disclosure is described with reference to FIG. 1.

[Characteristic Control in Third Embodiment]

In the first embodiment, the switching boundary is changed in the direction of increasing the engine load when the target EGR rate increases across the contour line shown in FIG. 3. As described above, this reason is because the time lag produces the period during which the actual EGR rate becomes excessive with respect to the target EGR rate. In the third embodiment, the ECU calculates the actual EGR rate during the acceleration operation to predict the time lag. The actual EGR rate is calculated based on an intake air amount, a supercharging pressure and an actual opening degree of the EGR valve, for example. The time lag corresponds to a time interval from a change point of the target EGR rate during the acceleration operation to an increase starting point of the actual EGR rate. Therefore, the actual EGR rate during the acceleration operation is calculated to obtain the above-described time interval, and the actual time lag Δt can be thereby predicted.

In the third embodiment, a position of the switching boundary is adjusted based on the predicted time lag Δt. FIG. 11 is a graph describing a method of adjusting the switching boundary in the third embodiment of the present disclosure. As shown in FIG. 11, the degree of the change of the switching boundary is increased as the predicted time lag Δt becomes larger. That is, the engine load at the switching boundary is set to a higher value as the predicted time lag Δt becomes larger. Therefore, it is possible to suppress the deterioration of the combustion state in the cylinder during the excessive period in accordance with the time lag Δt. It is to be noted that the engine load value when the time lag Δt equals zero as shown in FIG. 11 corresponds to the engine load value at the switching boundary during the steady operation shown in FIG. 4.

Other Embodiment

The above-described first to third embodiments are described on the assumption that the internal combustion engine is provided with an LPL-EGR device. However, the internal combustion engine may be provided with an HPL-EGR (high pressure loop-EGR) device instead of the LPL-EGR device. The internal combustion engine may be provided with a non-supercharging EGR device instead of the LPL-EGR device. The internal combustion engine may be provided with both of the LPL-EGR device and the HPL-EGR device. However, considering that influence by the time lag of the external EGR rate becomes the largest in the LPL-EGR device, the methods described in the above-described first to third embodiments are particularly effective in the internal combustion engine provided with the LPL-EGR device. 

What is claimed is:
 1. An internal combustion engine comprising: an exhaust gas system; an air intake system; an external EGR system for recirculating exhaust gas from the exhaust gas system to the air intake system; and a control device; wherein the control device further comprises: an EGR map the defines a relationship between an operating range defined by engine speed and engine load and a target value of external EGR rate, and has a predetermined partitioned range in which the target value is set to a highest value; and an operating angle map that defines a relationship between the operating range and an operating angle of an intake cam for driving an intake valve of the engine; wherein the operating angle is set to have a large operating angle being selected in a first region including a region corresponding to the predetermined partitioned range to close the intake valve in a first crank angle section; and a small operating angle being selected in a second region in which the engine load is higher than that of the first region to close the intake valve in a second crank angle section that is located nearer to a bottom dead center side than the first crank angle section; wherein the control device includes executable instruction stored in a microprocessor to: select the operating angle in accordance with the operating angle map when the engine operating state stays in a partitioned range having the equal target value in the EGR map; and change a boundary between the first region and the second region in a direction of increasing the engine load, and then select the operating angle in accordance with the operating angle map when the engine operating state is transferred from a partitioned range with the high target value to a partitioned range with the low target value in the EGR map and in the direction of increasing the engine load.
 2. The internal combustion engine according to claim 1, wherein the control device further includes executable instructions stored in the microprocessor to increase a degree of a change of the boundary as a change rate of an accelerator opening degree of the engine becomes larger when changing the boundary in the direction of increasing the engine load.
 3. The internal combustion engine according to claim 1, wherein the control device further includes executable instructions stored in the microprocessor to determine a time interval from a change point of the target value set in accordance with the EGR map to an increase starting point of an actual external EGR rate and increase the degree of the change of the boundary as the time interval is larger when changing the boundary in the direction of increasing the engine load.
 4. The internal combustion engine according to claim 1, further comprising a turbocharger including a compressor and a turbine; wherein the external EGR system recirculates the exhaust gas from a downstream side of the turbine to an upstream side of the compressor. 